Can't deprecate generics

[hadley]

While working on the evolution vignette (#143), we (well mostly Claude 😆) verified that the two existing ways to retire a generic's name each cover only half the problem:

• Plain alias (gen1 <- gen2, both exported): downstream method registrations keep working and both names dispatch (lab scenario gen-rename-alias), but callers get no deprecation signal.
• Wrapper function (gen1 <- function(x, ...) { .Deprecated("gen2"); gen2(x, ...) }): callers are warned, but a wrapper is not a generic, so every downstream method(gen1, ...) <- errors and those packages can't even be installed.

Instead I think we should provide a first-class function to help do this:

# pkgA, version 2
gen2 := new_generic("x")
gen1 := deprecated_generic(gen2)  # exported under the old name

The returned object is a closure with class S7_deprecated_generic that:

1. On call: signals a classed deprecation warning (probably once per session) and delegates to gen2() with all arguments.
2. On registration: as_generic() (and as_external_generic()) unwrap it, so method(gen1, MyClass) <- f registers on the shared gen2.
3. On print/introspection: identifies itself as deprecated and names the replacement.

Bonus properties:

• Mitigates Can't move generics to another package #729 for renames: a stale downstream package's deferred registration looks up the old name, finds the deprecated-generic object, and unwraps it — instead of library() failing.
• Also the right endpoint for a generic moved to another package (vignette("evolution")'s move recipe): NAMESPACE re-export while alive, deprecated_generic(pkgAcore::gen) when retiring the old home.

Related: #727 is the class-side counterpart (renaming a class through an alias); #726 (graceful re-sourcing) is about the same check-vs-continue tension at method registration time.

I think the default should use .Deprecated() and but provide a callback so you can use lifecycle style if you want. Also needs to work with no existing generic to declare it's deprecated with no replacement.